SEEPAGE 



95 



This movement is fast m a coarse sand and very slow in a clay- 

 loam of compact structure and m a clay. 



Percolation Prevents or Lessens Soil Erosion. — If water can 

 penetrate and peicolate thiousjh a soil easily, soil erosion is pre- 

 vented. A stubble field of heavy silt loam or clay loam on a hill- 

 side washes badly during a heavy rain because the water cannot 

 leadily soak into and percolate through it, and because the silt and 

 clay particles are easily carried away by running water. If such 

 a field were plowed so as to permit the water to enter the soil, 

 erosion may be checked or much lessened. 



Drainage Depends on Percolation. — When the ground is very 

 muddy the pore spaces in the soil are filled with water, in other 

 words, the soil is saturated or nearly so. If it were not possible 



Fig 39 — Diagram showing difference between percolation and seepage, A, the tilled 

 soil B, permeable subsoil through which free water percolates, C sandy gravel substratum 

 through which ground i^ater mo\es laterally W area at foot of slope kept wet by seepage 

 of water out from the upland, 0, place to put tile to catch the seepage water 



for this surplus water to pass down and away, workable soils 

 wotild soon become unfit for cultivation. It is through percolation 

 largely that soils rid themselves of any free or gravitational water 

 whenever opportunity be given it to flow away. 



Percolation Aided by Roots and Worms. — Many upland soils 

 are given better drainage through the penetration of roots into the 

 deep subsoil, and through the action of earthworms. When roots 

 of trees and of plants like alfalfa die there remain the openings 

 made by them. These openings, together with those made, by 

 earthworms, facilitate percolation. Checks and cracks in the soil 

 also favor this movement of soil water. 



Seepage is generally understood to mean slow lateral movement 

 of free soil water (Fig. 39). It is common in some locahties to 

 observe water seeping out of hillsides. Soils occurring on the 

 borders of marshes are often much wetter than the interior of the 

 marsh, owing to the seepage of free or gravitational water out 

 from the upland. 



